My commonly assigned U.S. Pat. No. 5,362,223 describes a plate sieve changer of a type in wide use for filtering molten plastic. In order to replace the sieve, the pivotal element or slide gate can be swung in a very short time, by means of a pressure-applying device, for example, so that a ram of the extruder feeding the plastic through the plate sieve can pass through without obstruction. The swinging motion allows changeover of the sieves as well. However, the seals which separate the melting channel from the pivotal element or slide gate have proven to be problematic. These seals should allow reliable and/or absolute sealing of the pivotal element even at high melting pressures of up to 400 bar at high melting temperatures and with low-viscosity molten plastic.
In addition, wear on the surfaces of the seals should be as slight as possible, so that practically absolute sealing is guaranteed even after frequent sieve replacements have taken place.
Although the use of elastic seals has been tried, they are not in widespread use due to their low degree of stability at the high temperatures indicated above.
Seal rings made of steel have been employed. These rings, relatively resistant to wear, begin to lose their sealing properties even after slight damage to the surface or the seat of the seal so that softer/tougher metals are in more widespread use, e.g. aluminum-bronzes, which can adapt more easily to slight unevenness.
The actual contact pressure can be set, for example, by means of set screws/screw threads. It is also possible to employ or interpose springs in order to achieve as constant a contact pressure as possible.
A hydraulic adjustment system having hydraulically-operated tappets distributed around the perimeter of the seal ring which influence the seal ring is known from DE-PS 19 19 269. A sealing body in the form of a supporting hydraulic ring space enclosed by metal bellows is known from DE-PS 30 43 217.
However, it has been seen that although the adjustability of the contact pressure to desired values is advantageous, and allows an adjustment to the temperature, the pressure and the viscosity of the molten plastic to the filtered, the adjustment of contact pressure by means of screws, screw threads and such like has proven to be complicated, while seals adjusted by means of a pressure-applying device are costly.